The expression given is,
[tex]x^2-x-20[/tex]Factorize the expression above
[tex]\begin{gathered} Break\text{ the expressions into two groups} \\ \left(x^2+4x\right)+\left(-5x-20\right) \\ Factorize\text{ out the common terms} \\ =x\left(x+4\right)-5\left(x+4\right) \\ \mathrm{Factor\:out\:common\:term\:}x+4 \\ =\left(x+4\right)\left(x-5\right) \end{gathered}[/tex]Hence, the answer is
[tex]\left(x+4\right)\left(x-5\right)[/tex]